API Client.Create & get client.ID

I require to add clients to CM from in-house database & get client.ID from CM, for the in-house database matchID
-so i use API Client.Create but...
-How do i get the CM.client.ID, without doing a get.client..list and then a xPath query or similar?
This method is restrictively slow as client list runs at 50k and the in-house database will be adding clients individually, and in bulk to start with.

- ideas? can the SOAPorPOST response have the ID or can a get.client.details be called with the email address (without an ID)?

thanks
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(apologies for the reposting)

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